Abstract
The granular layer is the input layer of the cerebellar cortex. It receives information through mossy fibers, which contact local granular layer interneurons (GLIs) and granular layer output neurons (granule cells). GLIs provide one of the first signal processing stages in the cerebellar cortex by exciting or inhibiting granule cells. Despite the importance of this early processing stage for later cerebellar computations, the responses of GLIs and the functional connections of mossy fibers with GLIs in awake animals are poorly understood. Here, we recorded GLIs and mossy fibers in the macaque ventral-paraflocculus (VPFL) during oculomotor tasks, providing the first full inventory of GLI responses in the VPFL of awake primates. We found that while mossy fiber responses are characterized by a linear monotonic relationship between firing rate and eye position, GLIs show complex response profiles characterized by “eye position fields” and single or double directional tunings. For the majority of GLIs, prominent features of their responses can be explained by assuming that a single GLI receives inputs from mossy fibers with similar or opposite directional preferences, and that these mossy fiber inputs influence GLI discharge through net excitatory or inhibitory pathways. Importantly, GLIs receiving mossy fiber inputs through these putative excitatory and inhibitory pathways show different firing properties, suggesting that they indeed correspond to two distinct classes of interneurons. We propose a new interpretation of the information flow through the cerebellar cortex granular layer, in which mossy fiber input patterns drive the responses of GLIs not only through excitatory but also through net inhibitory pathways, and that excited and inhibited GLIs can be identified based on their responses and their intrinsic properties.
Highlights
Influential theories of cerebellar cortex posit a fundamental role of granular layer local circuit neurons, collectively called Granular Layer Interneurons (GLIs), in the computations carried out by the cerebellar cortex [1,2,3]
We identified the three layers of the cerebellar cortex using standard criteria: i) the molecular layer was identified by the presence of complex spikes and the absence of large units, ii) the Purkinje cell layer was identified by the presence of Purkinje cells, identifiable by their simple spike pause (10 ms or more) following complex spikes, iii) the granular layer was identified based on its characteristic hashing activity, the absence of complex spikes and the presence of sparsely distributed neurons and mossy fibers
Our study provides the best characterization to date of the responses of individual GLIs in the awake animal, and offers clues about the functional connectivity between mossy fibers and GLIs
Summary
Influential theories of cerebellar cortex posit a fundamental role of granular layer local circuit neurons, collectively called Granular Layer Interneurons (GLIs), in the computations carried out by the cerebellar cortex [1,2,3]. These interneurons perform the first transformations of the input signals arriving in the cerebellar cortex. Of the GLIs contributing to these local circuit computations there are two different classes of neuron that are thought to play the largest roles. Golgi cells are inhibitory interneurons [4] thought to sample mossy fiber activity directly through their descending dendrites and indirectly via parallel fibers through their ascending dendrites (Figure 1), which allows them to provide both feedforward and feedback inhibition of granule cells and other GLIs
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